In an axial flow compressor, an axially split compressor case offers several advantages. The case can be installed around an inertia welded rotor and it also facilitates maintenance operations such as changing blades and vanes.
The axial split, however, requires that all rings inside the case be cut into pieces of 180 degrees or less. This requires that various new ways be found to fit the parts together For example, diametral tight fits cannot be used because segmented parts have no hoop strength.
Because of the aerodynamics of compressing air, the higher stages of the compressor operate at elevated temperatures on the order of 600.degree. centigrade. In aircraft where low weight is important, the casing is normally made of titanium which is of light weight. On the other hand the vanes and stator shrouds are usually made of nickel because of its higher strength Titanium has a low alpha (which is its coefficient of expansion) while nickel has a high alpha. Therefore, even when both are raised to an identical temperature, the nickel expands more than the titanium. Provision must be made to tolerate the differential expansion between the nickel vanes and shroud, and the titanium case to which they are secured.
Provisions must also be made to anti-rotate the vanes, this not being a significant problem in compressors which may use diametral tight fits.
In the interest of efficiency of the compressor it is desirable to prevent recirculation of air from the higher stages to the lower stages.
The vanes should be easily removable for maintenance and it is important to locate these vanes accurately in the axial direction to avoid interference with the adjacent rotating blades.